io: implement AsyncSeek for Empty

tokio/src/io/util/empty.rs

@@ -1,8 +1,8 @@
 use crate::io::util::poll_proceed_and_make_progress;
-use crate::io::{AsyncBufRead, AsyncRead, AsyncWrite, ReadBuf};
+use crate::io::{AsyncBufRead, AsyncRead, AsyncSeek, AsyncWrite, ReadBuf};
 
 use std::fmt;
-use std::io;
+use std::io::{self, SeekFrom};
 use std::pin::Pin;
 use std::task::{Context, Poll};
 
@@ -133,6 +133,18 @@ impl AsyncWrite for Empty {
     }
 }
 
+impl AsyncSeek for Empty {
+    fn start_seek(self: Pin<&mut Self>, _position: SeekFrom) -> io::Result<()> {
+        Ok(())
+    }
+
+    fn poll_complete(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<u64>> {
+        ready!(crate::trace::trace_leaf(cx));
+        ready!(poll_proceed_and_make_progress(cx));
+        Poll::Ready(Ok(0))
+    }
+}
+
 impl fmt::Debug for Empty {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.pad("Empty { .. }")

tokio/tests/io_util_empty.rs

@@ -1,5 +1,6 @@
 #![cfg(feature = "full")]
-use tokio::io::{AsyncBufReadExt, AsyncReadExt};
+use tokio::io::{AsyncBufReadExt, AsyncReadExt, AsyncSeekExt};
+use tokio_test::assert_ok;
 
 #[tokio::test]
 async fn empty_read_is_cooperative() {
@@ -30,3 +31,16 @@ async fn empty_buf_reads_are_cooperative() {
         _ = tokio::task::yield_now() => {}
     }
 }
+
+#[tokio::test]
+async fn empty_seek() {
+    use std::io::SeekFrom;
+
+    let mut empty = tokio::io::empty();
+
+    let pos = assert_ok!(empty.seek(SeekFrom::Start(0)).await);
+    assert_eq!(pos, 0);
+
+    let pos = assert_ok!(empty.seek(SeekFrom::Start(8)).await);
+    assert_eq!(pos, 0);
+}